1. Field of the Invention
The present invention relates to a curved tube vibrating flow meter, and more particularly, to thermal stress compensation in a curved tube vibrating flow meter.
2. Statement of the Problem
Vibrating tube sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating tube or tubes that contains a flowing material. Properties associated with the material in the tube, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more tubes that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each tube may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a tube is excited in one or more vibration modes as a material flows through the tube, and motion of the tube is measured at points spaced along the tube. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Frequency of the vibrational response may be measured by a single transducer, wherein the frequency is used to determine the density of material in the meter. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. Vibrating flow meters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow tubes that are vibrated in order to measure a fluid.
Vibratory meters may be used with hot or cold flow materials. However, thermal stress is induced in a flow meter when the meter's flow tube or tubes are at a different temperature than other parts of the meter assembly. For instance, when hot fluid is suddenly introduced into a cold meter, the flow tube tries to expand in length but is constrained by the (relatively) cold case. This situation is known as thermal shock. In the more common steady-state situation, the fluid is hot but the ambient environment is cold and a temperature gradient exists across portions of the meter as a result.
In a typical flowmeter, the thermal expansion or contraction may be constrained or prevented by the design of the flowmeter. The thermal differences in the meter therefore create thermal stress on the flow tube or tubes of the meter. For example, a tube's axial expansion or contraction may be constrained by the meter case.
U.S. Pat. No. 6,327,915 to Van Cleve discloses a straight tube Coriolis flowmeter including a balance bar and temperature sensors S1-S4. A single temperature sensor S4 is used to measure the temperature of the case. The network of temperature sensors provides temperature information that is used to perform thermal stress compensation, wherein temperature changes will cause compression or tension forces on the vibrating tube, affecting the resonant frequency of the straight flow tube.
A straight tube vibrating meter, due to its shape, does not have a bending stress, as is present in a curved tube meter. A straight tube vibrating meter does not require multiple case temperature measurements or case temperature measurements at specific locations which will affect a tube bending stress.
It is well known that Coriolis flow meters having dual-curved tubes are of the highest accuracy in terms of flow measurement. They are also of high accuracy in density measurement, but not as accurate as some currently produced densitometers having a single straight tube.
However, despite their advantages, straight tube densitometers have disadvantages. The straight and relatively rigid flow tubes cannot freely expand or contract because of the constraint of the case and other components. The resulting thermal compression or tension on a straight tube vibrating densitometer will change the resonant frequency. This effect of heating or cooling, with the resulting constrained expansion or contraction of the flowmeter, is called thermal stress.
Single straight tube densitometers achieve their accuracy in part by inclusion of a bellows at either end of the active flow tube, wherein the bellows allows thermal expansion and contraction of the flow meter assembly. The bellows therefore isolate the flow tube from the thermal stress that could otherwise change the frequency of the flow tube and thus impair the accuracy of the meter.
Bellows have several disadvantages. First, they limit the fluid pressure rating of the meter. Second, they impair the ability of the meter to be rated as sanitary, as the bellows will trap and retain flow material after a flow is stopped. Third, the bellows require more expensive and complicated construction and therefore have a higher cost.